The fire mosaic (no single right fire)

You can read how long ago a patch of this country last burnt from a few paces off, without ever seeing a flame. Straight eucalypt trunks standing over open, grassy floor have been through fire recently and lightly. A heath ablaze with spring peas and boronias answers a burn with everything it has. And where woody scrub is closing over what was once open woodland, the ground has gone a long time without fire. Each patch is a clock, set running the day it last burnt, and the whole landscape is a wall of clocks all showing different hours.

Fuel drives the clock. Straight after a fire the ground lies nearly bare, almost nothing to carry a flame. Then the litter piles up, leaf and bark and twig, slow to rot on poor soil, and the understorey thickens and knits together. But it does not build without limit. Fuel accumulates fast for a decade or so and then falls away again as the shrub layer senesces (Dixon et al. 2018). So a patch cycles through phases as it ages: open, then thickening, then rank and closed. A different cast of plants and animals suits each phase. The open floor after a burn is a wildflower garden; the middle years grow the dense cover some birds need; the long-unburnt stand is something else again.

Lay fire across the country patchily rather than all at once, and you hold all those phases at the same time: last year’s burn beside a five-year-old thicket beside ground untouched for twenty. That patchwork is the fire mosaic, and the diversity of the whole is the diversity of the patchwork — every phase kept somewhere on the map at once. Take the ground parrot, which needs heath of one particular age since fire; a healthy mosaic holds a patch at about its stage, and another coming on behind.

The hard local proof of how much this matters comes from a long-term burning experiment in the region’s heath and woodland, run for close on half a century at Beerwah Scientific Area and at Mt Bilewilam in the Cooloola Recreation Reserve, where researchers have manipulated how often plots burn and watched the plants for years (Dooley, Lewis and Schmidt, 2023). It found that fire frequency is a master switch on the understorey. Plots burnt regularly kept an open, herb-rich floor; in the heath left long unburnt the open ground vanished under a dense blanket of coral fern, and diversity fell away beneath it. That last case stores more carbon and loses the very diversity a burn maintains. The plain finding, dug out with drip-torches and quadrats and a great deal of patience, is that no single setting suited everything.

The whole coast turns on that corollary. There is no one right fire, because the coast is a gradient of many worlds, and each world along it is held in place by a different flame. The rainforest takes none. The wet eucalypt forest takes a little, now and then. The dry grassy woodland takes a cool burn every few years. The wallum heath is the most fire-hungry of all, its shrubs banking seed for the burn and flushing their flowers after it. Match the fire to the ground and you keep every world; get it wrong in either direction and you lose one.

This is also why the surest way to drain the country of variety is a fire regime too uniform — the same interval and the same heat laid on everywhere. Uniform fire is not the safe middle course it sounds like. It settles the whole landscape onto a single point of the cycle, and every species tuned to a different point quietly falls out. Diversity rides on the mosaic, on keeping every stage available somewhere all the time; no single perfect fire, applied evenly, will do it.

None of this is a modern discovery. Aboriginal people worked this country as exactly such a fine-grained patchwork of recently burnt and longer-unburnt ground for tens of thousands of years, a landscape gardened with fire, a case argued at book length by Bill Gammage and set at the centre of Australian vegetation history by the ecologist David Bowman, though the grander versions of the thesis have been vigorously debated since. Putting deliberate, well-matched fire back is now one of the central tasks of caring for the coast, and one of the hardest, because both neglect and heavy-handedness are degradation pathways in their own right (Furlaud et al. 2021). You do not keep the mosaic by leaving the country alone. It is kept by a hand that reads each patch and gives it the flame it was built around, knowing all the while that the fire the rainforest edge needs is the wrong fire for the heath a ridge away.